The present invention relates generally to a combined plant including a steam turbine and a gas turbine connected together by a single shaft, the steam turbine being driven by the steam generated by using the waste heat from the gas turbine as a heat source, and more particularly to an apparatus for and a method of effecting cooling so as to prevent overheating of the steam turbine at the time of plant startup.
In this type of single-shaft combined plant, the steam turbine and gas turbine can be started and accelerated at the same time. Thus, this type of plant offers the advantage that it is possible to shorten the time required for achieving startup as compared with a multiple-shaft type combined plant in which the steam turbine and gas turbine have separate shafts.
However, in this type of single-shaft combined plant, feeding of steam to the steam turbine is not obtainable until the gas turbine is first accelerated and its exhaust gases are led to a waste heat recovery boiler to generate steam by using the exhaust gases as a heat source. As the consequence, the steam turbine may be overheated by a windage loss (loss of power provided by the agitation of gases within the steam turbine) until steam is fed to the steam turbine. Although the steam turbine is arranged to have its inner pressure reduced with a vacuum pump for a condenser, the inner pressure substantially approximates the atmospheric pressure at the time of plant startup. In addition, in order to maintain the condenser in vacuum, the gland sealing portion of the condenser is supplied with sealing steam having a high temperature of about 300.degree. C., and the sealing steam flows through the gland sealing portion into the steam turbine. In particular, this steam remarkably heats the low pressure final stage of the turbine or stages near it. Moreover, since the turbine has elongated rotor blades at the final stage and stages near it, centrifugal stresses developing at the roots of the blades are higher at the final stage and stages near it than at an initial stage of the turbine. For this reason, if the temperature in this part of the turbine shows a marked rise in temperature, the material would undesirably be greatly reduced in strength.
To cope with the aforesaid problem, a known method is proposed, for example, in U.S. Pat. No. 4,519,207. In this prior method, an ancillary steam source is provided for the purposes of a cooling operation, and the cooling steam generated by the ancillary steam source is introduced into the low pressure final stages or similar stages of the steam turbine, thereby preventing overheating thereof.
The above-described method according to the prior art, however, suffers from the problem of causing an increase in plant construction costs incurred by the addition of the ancillary steam source and associated systems. Furthermore, since it is necessary to supply ancillary steam through a line independent of the single combined shaft system, there is a problem in that it is impossible to perform "black start" of the single combined shaft system alone.